1. Field of the Invention
Apparatuses and methods consistent with exemplary embodiments relate to a polarization conversion optical system and a liquid crystal projection apparatus including the polarization conversion optical system.
2. Description of the Related Art
Illuminating optical systems used in liquid crystal projectors (liquid crystal projection apparatuses) generally include a polarization conversion optical system which controls a polarization direction of illuminating light in order to improve an illuminating efficiency.
Japanese Patent Laid-open publication No. 2002-797317 (reference 1) discloses a polarization conversion optical system in which a plurality of polarization-separating prisms are disposed in front of a light source.
Japanese Patent Publication No. 3555610 (reference 2) discloses a polarization conversion optical system including a deviation prism, a lens array, a polarization separating prism array, and an exiting lens.
However, the above conventional polarization conversion optical systems and the liquid crystal projection apparatus have problems that both technologies disclosed in references 1 and 2 split polarization by transmitting a polarization component along an optical axis and reflecting the other polarization component in a direction perpendicular to the optical axis with one (a first polarization separating prism) of a plurality of polarization separating prisms which are arranged in parallel. In addition, the polarization component reflected by the first polarization separating prism is reflected in an optical axis direction by another polarization separating prism (hereinafter, a second polarization separating prism) which is adjacent to the first polarization separating prism. In addition, the polarization direction is rotated 90° by a polarization conversion device such as a half-wave plate (half-phase difference plate) to convert the polarization direction, and one of the lights transmitted through the first and second polarization separating prisms is controlled to be in the same polarization direction as the other to be output.
Therefore, in reference 1, for example, the pair of the first and second polarization separating prisms is replaced in such a way that two adjacent first polarization separating prisms face each other, a bundle of illuminating rays is incident on a range of the two first polarization separating prisms which are arranged on a center portion of the incident illuminating flux so as to irradiate the illuminating flux, the polarization direction of which is controlled, having a diameter that is twice of the incident illuminating flux. In addition, luminance distribution of the illuminating flux is made uniform by a rod integrator, and then, the illuminating flux is irradiated onto a liquid crystal panel. In this case, the illuminating flux is irradiated to the liquid crystal panel after the diameter of the illuminating flux incident on the first polarization separating prism is enlarged to twice the diameter, and accordingly, the light source emits light of high intensity in order to ensure a sufficient quantity of light on a projection screen. In particular, when a surface light source such as a light emitting diode (LED) is used, a sufficient quantity of light may not be ensured.
In addition, according to reference 2, in an array of the first and second polarization separating prisms and the polarization conversion optical system, the first polarization separating prism and the second polarization separating prism are alternately disposed in parallel in a direction perpendicular to the optical axis. The above polarization conversion optical system substantially has the same structure as that of reference 1, and thus, a cross-sectional area of the output flux is twice of the cross-sectional area of the incident flux. Therefore, the illuminating flux that is incident on the polarization separating prism array is condensed by the deviation prism and the lens array, and then, is incident on the first polarization separating prisms which are arranged alternately.
Therefore, the diameter of the flux emitted from the light source may be equal to the diameter of the flux condensed on the exiting lens of the polarization separating prism array; however, the optical system including the deviation prism, the lens array, and the exiting lens is required. Thus, a lot of quantity of light is lost, and the structure of the optical system becomes complex due to a lot of components.